Fallback assistance information

ABSTRACT

Apparatuses, methods, and systems are disclosed for providing fallback assistance information to a RAN node. One apparatus includes a transceiver that sends a service request, wherein the service request requires fallback to at least one of: a different RAT and a different system generation, wherein the service request indicates that service fallback is required and receives a connection release message, where the connection release message includes redirection information for the service fallback, the redirection information including a target RAT cell and a target system. The apparatus includes a processor that selecting a NAS protocol from a plurality of NAS protocols supported by the remote unit, wherein the selection is based on the target system and connects to the target RAT and the target system using a procedure of the selected NAS protocol.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.16/197,122 entitled “Fallback Assistance Information” and filed on Nov.20, 2018 for Genadi Velev, Prateek Basu Mallick, Joachim Lohr, and RaviKuchibhotla, which application claims priority to U.S. ProvisionalPatent Application No. 62/588,846 entitled “Fallback AssistanceInformation” and filed on Nov. 20, 2017 for Genadi Velev, Prateek BasuMallick, Joachim Lohr, and Ravi Kuchibhotla, which applications areincorporated herein by reference.

FIELD

The subject matter disclosed herein relates generally to wirelesscommunications and more particularly relates to providing fallbackassistance information to a RAN node.

BACKGROUND

The following abbreviations are herewith defined, at least some of whichare referred to within the following description: Third GenerationPartnership Project (“3GPP”), Positive-Acknowledgment (“ACK”),Autonomous Uplink (“AUL”), AUL Downlink Feedback Information(“AUL-DFP”), Binary Phase Shift Keying (“BPSK”), Clear ChannelAssessment (“CCA”), Cyclic Prefix (“CP”), Cyclical Redundancy Check(“CRC”), Channel State Information (“CSI”), Common Search Space (“CSS”),Discrete Fourier Transform Spread (“DFTS”), Downlink Control Information(“DCI”), Downlink (“DL”), Downlink Pilot Time Slot (“DwPTS”), EnhancedClear Channel Assessment (“eCCA”), Enhanced Licensed Assisted Access(“eLAA”), Enhanced Mobile Broadband (“eMBB”), Evolved Node B (“eNB”),European Telecommunications Standards Institute (“ETSI”), Frame BasedEquipment (“FBE”), Frequency Division Duplex (“FDD”), Frequency DivisionMultiple Access (“FDMA”), Frequency Division Orthogonal Cover Code(“FD-OCC”), Guard Period (“GP”), Hybrid Automatic Repeat Request(“HARQ”), Internet-of-Things (“IoT”), Licensed Assisted Access (“LAA”),Load Based Equipment (“LBE”), Listen-Before-Talk (“LBT”), Long TermEvolution (“LTE”), Multiple Access (“MA”), Modulation Coding Scheme(“MCS”), Machine Type Communication (“MTC”), Multiple Input MultipleOutput (“MIMO”), Multi User Shared Access (“MUSA”), Narrowband (“NB”),Negative-Acknowledgment (“NACK”) or (“NAK”), Next Generation Node B(“gNB”), New Data Indicator (“NDI”), Non-Orthogonal Multiple Access(“NOMA”), Orthogonal Frequency Division Multiplexing (“OFDM”), PrimaryCell (“PCell”), Physical Broadcast Channel (“PBCH”), Physical DownlinkControl Channel (“PDCCH”), Physical Downlink Shared Channel (“PDSCH”),Pattern Division Multiple Access (“PDMA”), Physical Hybrid ARQ IndicatorChannel (“PHICH”), Physical Random Access Channel (“PRACH”), PhysicalResource Block (“PRB”), Physical Uplink Control Channel (“PUCCH”),Physical Uplink Shared Channel (“PUSCH”), Quality of Service (“QoS”),Quadrature Phase Shift Keying (“QPSK”), Radio Resource Control (“RRC”),Random Access Procedure (“RACH”), Random Access Response (“RAR”), RadioNetwork Temporary Identifier (“RNTI”), Reference Signal (“RS”),Remaining Minimum System Information (“RMSI”), Resource Block Assignment(“RBA”), Resource Spread Multiple Access (“RSMA”), Round Trip Time(“RTT”), Receive (“RX”), Sparse Code Multiple Access (“SCMA”),Scheduling Request (“SR”), Single Carrier Frequency Division MultipleAccess (“SC-FDMA”), Secondary Cell (“SCell”), Shared Channel (“SCH”),Signal-to-Interference-Plus-Noise Ratio (“SINR”), System InformationBlock (“SIB”), Synchronization Signal (“SS”), Transport Block (“TB”),Transport Block Size (“TBS”), Time-Division Duplex (“TDD”), TimeDivision Multiplex (“TDM”), Time Division Orthogonal Cover Code(“TD-OCC”), Transmission Time Interval (“TTI”), Transmit (“TX”), UplinkControl Information (“UCI”), User Entity/Equipment (Mobile Terminal)(“UE”), Uplink (“UL”), Universal Mobile Telecommunications System(“UMTS”), Uplink Pilot Time Slot (“UpPTS”), Ultra-reliability andLow-latency Communications (“URLLC”), and Worldwide Interoperability forMicrowave Access (“WiMAX”). As used herein, “HARQ-ACK” may representcollectively the Positive Acknowledge (“ACK”) and the NegativeAcknowledge (“NACK”). ACK means that a TB is correctly received whileNACK (or NAK) means a TB is erroneously received.

In certain wireless communications networks, a UE operating in a mobilecommunication network requires a change to different system, such asdifferent network core (e.g., system generation) and/or radio accesstechnology in order to receive the service in the mobile communicationnetwork. Fallback procedures are commonly used to allow interoperabilityof various services used by the remote unit 105, conventional fallbackprocedures involve the switch from a packet-switched domain(“PS-domain”) to a circuit-switched domain (“CS-domain”). Moreover,conventional fallback procedures failed to inform the RAN and UE oftarget systems and RATS, thereby leading to situations where the UE mayconnect to the wrong system during the fallback procedure.

BRIEF SUMMARY

Methods for providing fallback assistance information to a RAN node aredisclosed. Apparatuses and systems also perform the functions of themethods.

One method (e.g., of a UE) for providing fallback assistance informationto a RAN node includes sending a service request, wherein the servicerequest requires fallback to at least one of: a different RAT and adifferent CN. The method includes receiving, at the remote unit, aconnection release message, where the connection release messageincludes redirection information for the service fallback, theredirection information including a target CN. The method includesselecting a NAS procedure based on the target CN and connecting to thetarget CN using the selected NAS procedure.

Another method (e.g., of a RAN node) for providing fallback assistanceinformation includes receiving a first message from a network functionin a first core network, wherein the first message indicates servicefallback of a remote unit connected to the RAN node and indicates atarget CN. The method includes creating service fallback parameters forthe remote unit. The method includes sending a connection releasemessage to the remote unit, where the connection release messageincludes redirection information for the service fallback, theredirection information including the target CN

Another method (e.g., of an AMF in a first core network) for providingfallback assistance information includes receiving a service requestfrom a remote unit, wherein the service request is for a service whichrequires fallback to at least one of: a different RAT and a differentCN. The method includes identifying, at the network function, at leastone of: a target RAT and a target CN, based on at least one of: therequested service, remote unit capabilities and network configuration.The method includes indicating, to a RAN node, the at least one of: atarget RAT and a target CN, based on the requested service, wherein theRAN node performs fallback with the remote unit based on the at leastone of: a target RAT and a target CN.

BRIEF DESCRIPTION OF THE DRAWINGS

A more particular description of the embodiments briefly described abovewill be rendered by reference to specific embodiments that areillustrated in the appended drawings. Understanding that these drawingsdepict only some embodiments and are not therefore to be considered tobe limiting of scope, the embodiments will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings, in which:

FIG. 1 is a schematic block diagram illustrating one embodiment of awireless communication system for providing fallback assistanceinformation to a RAN node;

FIG. 2 is a block diagram illustrating one embodiment of a procedure forproviding fallback assistance information to a RAN node;

FIG. 3 is a block diagram illustrating another procedure for providingfallback assistance information to a RAN node;

FIG. 4 is a diagram illustrating one embodiment of a connection releasemessage that may be used for providing fallback assistance informationto a RAN node;

FIG. 5 is a schematic block diagram illustrating one embodiment of auser equipment apparatus that may be used for providing fallbackassistance information to a RAN node;

FIG. 6 is a schematic block diagram illustrating one embodiment of a RANapparatus that may be used for providing fallback assistance informationto a RAN node;

FIG. 7 is a schematic block diagram illustrating one embodiment of anetwork function apparatus that may be used for providing fallbackassistance information to a RAN node;

FIG. 8 is a schematic block diagram illustrating a first embodiment of amethod for providing fallback assistance information to a RAN node;

FIG. 9 is a schematic block diagram illustrating a second embodiment ofa method be used for providing fallback assistance information to a RANnode; and

FIG. 10 is a schematic block diagram illustrating a third embodiment ofa method for providing fallback assistance information to a RAN node.

DETAILED DESCRIPTION

As will be appreciated by one skilled in the art, aspects of theembodiments may be embodied as a system, apparatus, method, or programproduct. Accordingly, embodiments may take the form of an entirelyhardware embodiment, an entirely software embodiment (includingfirmware, resident software, micro-code, etc.) or an embodimentcombining software and hardware aspects that may all generally bereferred to herein as a “circuit,” “module” or “system.” Furthermore,embodiments may take the form of a program product embodied in one ormore computer readable storage devices storing machine readable code,computer readable code, and/or program code, referred hereafter as code.The storage devices may be tangible, non-transitory, and/ornon-transmission. The storage devices may not embody signals. In acertain embodiment, the storage devices only employ signals foraccessing code.

Any combination of one or more computer readable medium may be utilized.The computer readable medium may be a computer readable storage medium.The computer readable storage medium may be a storage device storing thecode. The storage device may be, for example, but not limited to, anelectronic, magnetic, optical, electromagnetic, infrared, holographic,micromechanical, or semiconductor system, apparatus, or device, or anysuitable combination of the foregoing.

More specific examples (a non-exhaustive list) of the storage devicewould include the following: an electrical connection having one or morewires, a portable computer diskette, a hard disk, a random-access memory(“RAM”), a read-only memory (“ROM”), an erasable programmable read-onlymemory (“EPROM” or Flash memory), a portable compact disc read-onlymemory (“CD-ROM”), an optical storage device, a magnetic storage device,or any suitable combination of the foregoing. In the context of thisdocument, a computer readable storage medium may be any tangible mediumthat can contain, or store a program for use by or in connection with aninstruction execution system, apparatus, or device.

Code for carrying out operations for embodiments may be any number oflines and may be written in any combination of one or more programminglanguages including an object-oriented programming language such asPython, Ruby, Java, Smalltalk, C++, or the like, and conventionalprocedural programming languages, such as the “C” programming language,or the like, and/or machine languages such as assembly languages. Thecode may execute entirely on the user's computer, partly on the user'scomputer, as a stand-alone software package, partly on the user'scomputer and partly on a remote computer or entirely on the remotecomputer or server. In the latter scenario, the remote computer may beconnected to the user's computer through any type of network, includinga local area network (“LAN”) or a wide area network (“WAN”), or theconnection may be made to an external computer (for example, through theInternet using an Internet Service Provider).

Reference throughout this specification to “one embodiment,” “anembodiment,” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment. Thus, appearances of the phrases“in one embodiment,” “in an embodiment,” and similar language throughoutthis specification may, but do not necessarily, all refer to the sameembodiment, but mean “one or more but not all embodiments” unlessexpressly specified otherwise. The terms “including,” “comprising,”“having,” and variations thereof mean “including but not limited to,”unless expressly specified otherwise. An enumerated listing of itemsdoes not imply that any or all of the items are mutually exclusive,unless expressly specified otherwise. The terms “a,” “an,” and “the”also refer to “one or more” unless expressly specified otherwise.

Furthermore, the described features, structures, or characteristics ofthe embodiments may be combined in any suitable manner. In the followingdescription, numerous specific details are provided, such as examples ofprogramming, software modules, user selections, network transactions,database queries, database structures, hardware modules, hardwarecircuits, hardware chips, etc., to provide a thorough understanding ofembodiments. One skilled in the relevant art will recognize, however,that embodiments may be practiced without one or more of the specificdetails, or with other methods, components, materials, and so forth. Inother instances, well-known structures, materials, or operations are notshown or described in detail to avoid obscuring aspects of anembodiment.

Aspects of the embodiments are described below with reference toschematic flowchart diagrams and/or schematic block diagrams of methods,apparatuses, systems, and program products according to embodiments. Itwill be understood that each block of the schematic flowchart diagramsand/or schematic block diagrams, and combinations of blocks in theschematic flowchart diagrams and/or schematic block diagrams, can beimplemented by code. The code may be provided to a processor of ageneral-purpose computer, special purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions, which execute via the processor of the computer orother programmable data processing apparatus, create means forimplementing the functions/acts specified in the schematic flowchartdiagrams and/or schematic block diagrams block or blocks.

The code may also be stored in a storage device that can direct acomputer, other programmable data processing apparatus, or other devicesto function in a particular manner, such that the instructions stored inthe storage device produce an article of manufacture includinginstructions which implement the function/act specified in the schematicflowchart diagrams and/or schematic block diagrams block or blocks.

The code may also be loaded onto a computer, other programmable dataprocessing apparatus, or other devices to cause a series of operationalsteps to be performed on the computer, other programmable apparatus orother devices to produce a computer implemented process such that thecode which execute on the computer or other programmable apparatusprovide processes for implementing the functions/acts specified in theflowchart and/or block diagram block or blocks.

The schematic flowchart diagrams and/or schematic block diagrams in theFigures illustrate the architecture, functionality, and operation ofpossible implementations of apparatuses, systems, methods, and programproducts according to various embodiments. In this regard, each block inthe schematic flowchart diagrams and/or schematic block diagrams mayrepresent a module, segment, or portion of code, which includes one ormore executable instructions of the code for implementing the specifiedlogical function(s).

It should also be noted that, in some alternative implementations, thefunctions noted in the block may occur out of the order noted in theFigures. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. Other steps and methods may be conceived that are equivalentin function, logic, or effect to one or more blocks, or portionsthereof, of the illustrated Figures.

Although various arrow types and line types may be employed in theflowchart and/or block diagrams, they are understood not to limit thescope of the corresponding embodiments. Indeed, some arrows or otherconnectors may be used to indicate only the logical flow of the depictedembodiment. For instance, an arrow may indicate a waiting or monitoringperiod of unspecified duration between enumerated steps of the depictedembodiment. It will also be noted that each block of the block diagramsand/or flowchart diagrams, and combinations of blocks in the blockdiagrams and/or flowchart diagrams, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts, or combinations of special purpose hardware and code.

The description of elements in each figure may refer to elements ofproceeding figures. Like numbers refer to like elements in all figures,including alternate embodiments of like elements.

Disclosed herein are methods for service fallback procedure includingthe steps of: the CN providing fallback assistance information to theRAN node (e.g., a gNB), and the RAN node deciding about (1) the mobilitymechanism (IDLE or CONNECTED mode mobility) and (2) the target RAT, andif needed, target CN for the fallback procedure. Here, the fallbackassistance information may include type of mobility (e.g., whetherCONNECTED mode or IDLE mode mobility is preferred based on theconfiguration in the CN) and/or target core network(s) and/orCriticality requirements. Note that if the RAN node performs IDLE statemobility, then the UE considers the indicated target core network wheninitiating NAS procedure in the target cell. In various embodiments, theNAS procedure in the target CN may either 1) implicit include also arequest for the particular service; or 2) be an independent NASprocedure for the requested service.

In one example, the UE has a pending IMS emergency session request(e.g., voice) from the upper layers. If the AMF has indicated supportfor emergency services using fallback via the Registration Acceptmessage for the current RAT, then the UE sends a Service Request messageindicating that it requires emergency services fallback.

After receiving the Service Request for Emergency Fallback, the AMFtriggers N2 procedure resulting in either CONNECTED state mobility(Handover procedure) or IDLE state mobility (redirection) to eitherE-UTRA/5GC or to E-UTRAN/EPC depending on factors such as N26availability, network configuration and radio conditions. Here, the 5GCtriggers a request for Emergency Services Fallback by executing an NG-APprocedure in which it indicates to NG-RAN that this is a fallback foremergency services. In the N2 procedure, the AMF, based on the supportof Emergency Services in EPC or SGC, may indicate the target CN for theRAN node to know whether inter-RAT fallback or inter-system fallback isto be performed. The target CN indicated in the N2 procedure is alsoconveyed to the UE in order to be able to perform the appropriate NASprocedures (S1 or N1 Mode). When AMF initiates Redirection for UE(s)that have been successfully authenticated, AMF includes the securitycontext in the request to trigger fallback towards NG-RAN.

Based on the target CN indicated in the N2 request for EmergencyFallback, one of the following procedures is executed by NG-RAN: (1) ifUE is currently camped on NR, then the NG-RAN performs fallback (e.g.,initiates handover or redirection) to a 5GC-connected E-UTRAN cell; or(2) the NG-RAN initiates handover or redirection to E-UTRAN connected toEPS. Here, the NG-RAN uses the security context provided by the AMF tosecure the redirection procedure. One example of performing fallbackincludes the NG-RAN initiating handover or redirection to E-UTRAN.

Note that if the redirection procedure is used, then the target CN isalso conveyed to the UE in order to be able to perform the appropriateNAS procedures (S1 or N1 Mode). After handover to the target cell the UEestablishes a PDU Session/PDN connection for IMS emergency services andperforms the IMS procedures for establishment of an IMS emergencysession (e.g., voice).

FIG. 1 depicts an embodiment of a wireless communication system 100 forproviding fallback assistance information to a RAN node. In oneembodiment, the wireless communication system 100 includes at least oneremote unit 105, a first access network 120 containing at least one baseunit 110, a second access network 125 containing at least one base unit110, wireless communication links 115 between remote unit 105 and baseunit 110, a first core network 130, and a second core network 140. Eventhough a specific number of remote units 105, access networks 120, 125,base units 110, wireless communication links 115, and core networks 130,140 are depicted in FIG. 1, one of skill in the art will recognize thatany number of remote units 105, access networks 120, 125, base units110, wireless communication links 115, and core networks 130, 140 may beincluded in the wireless communication system 100. In variousembodiments, the access networks 120, 125 may contain one or more WLAN(e.g., Wi-Fi™) access points (“APs”). Here, the first access network120, second access network 125, first core network 130 and second corenetwork 140 belong to the same mobile communication network (e.g., thesame PLMN).

In one implementation, the wireless communication system 100 iscompliant with the 5G system and the LTE system specified in the 3GPPspecifications. More generally, however, the wireless communicationsystem 100 may implement some other open or proprietary communicationnetwork, for example, WiMAX, among other networks. The presentdisclosure is not intended to be limited to the implementation of anyparticular wireless communication system architectures or protocols.

In one embodiment, the remote units 105 may include computing devices,such as desktop computers, laptop computers, personal digital assistants(“PDAs”), tablet computers, smart phones, smart televisions (e.g.,televisions connected to the Internet), smart appliances (e.g.,appliances connected to the Internet), set-top boxes, game consoles,security systems (including security cameras), vehicle on-boardcomputers, network devices (e.g., routers, switches, modems), or thelike. In some embodiments, the remote units 105 include wearabledevices, such as smart watches, fitness bands, optical head-mounteddisplays, or the like. Moreover, the remote units 105 may be referred toas subscriber units, mobiles, mobile stations, users, terminals, mobileterminals, fixed terminals, subscriber stations, UEs, user terminals, adevice, or by other terminology used in the art. The remote units 105may communicate directly with one or more of the base units 110 viauplink (“UL”) and downlink (“DL”) communication signals. Furthermore,the UL and DL communication signals may be carried over the wirelesscommunication links 115.

In some embodiments, the remote units 105 communicate with a remote host180 (for example, an application server) via a data path that passesthrough one of the core networks 130 and 140 and also passes through thedata network 175. For example, a remote unit 105 may establish a PDUsession (or similar data connection) to the data network 175 via thefirst core network 130. The first core network 130 then relays trafficbetween the remote unit 105 and the remote host 180 using the PDUsession. As another example, a remote unit 105 may establish a PDNconnection to the data network 175 via the second core network 140. Thesecond core network 140 then relays traffic between the remote unit 105and the remote host 180 using the PDN connection.

The base units 110 may be distributed over a geographic region. Incertain embodiments, a base unit 110 may also be referred to as anaccess terminal, an access point, a base, a base station, a Node-B, aneNB, a gNB, a Home Node-B, a relay node, a device, or by any otherterminology used in the art. The base units 110 are generally part of aradio access network (“RAN”), such as the first access network 120(e.g., NG-RAN) and/or the second access network 125 (e.g., E-UTRAN),that may include one or more controllers communicably coupled to one ormore corresponding base units 110. These and other elements of the radioaccess network are not illustrated, but are well known generally bythose having ordinary skill in the art.

The base units 110 may serve a number of remote units 105 within aserving area, for example, a cell or a cell sector via a wirelesscommunication link 115. The base units 110 may communicate directly withone or more of the remote units 105 via communication signals.Generally, the base units 110 transmit downlink (“DL”) communicationsignals to serve the remote units 105 in the time, frequency, and/orspatial domain. Furthermore, the DL communication signals may be carriedover the wireless communication links 115. The wireless communicationlinks 115 may be any suitable carrier in licensed or unlicensed radiospectrum. The wireless communication links 115 facilitate communicationbetween one or more of the remote units 105 and/or one or more of thebase units 110.

As depicted, the wireless communication system 100 includes both a firstcore network 130 and a second core network 140. Here, the first corenetwork 130 and second core network 140 are part of the same PLMN. Eachof the first core network 130 and second core network 140 is associatedwith a different “system generation” of the mobile communicationnetwork. Moreover, some services are available in the first core network130 that are not available in the second core network 140, and viceversa. Accordingly, in some situations the remote unit 105 may need tofallback from the first core network 130 to the second core network 140in order to access certain services.

The first core network 130 includes an Access and Mobility ManagementFunction (“AMF”) 131, a SMF 133, a UPF 135, and a UDM 137. Additionally,the second core network 140 includes a Mobility Management Entity(“MME”) 141, a Serving Gateway (“SGW”) 143, a Packet Gateway (“PGW”)145, and a Home Subscriber Server (“HSS”) 147. In some embodiments, theUDM 137 and HSS 147 may be co-located and/or may be a single entityshared by the first core network 130 and second core network 140.Although specific numbers and types of network functions are depicted inFIG. 1, one of skill in the art will recognize that any number and typeof network functions may be included in the mobile core networks 130 and140. While the second core network 140 is depicted as including an IPmultimedia subsystem (“IMS”) 150, in other embodiments, the IMS 150 maybe separate from the second core network 140.

In one embodiment, the first core network 130 is a fifth-generation corenetwork (“5GC”). Such a first core network 130 may be accessed using theNew Radio (“NR”) Radio Access Technology (“RAT”) or the LTE RAT. In oneembodiment, the second core network 140 is an Evolved Packet Core(“EPC”) or similar fourth-generation core network. Such a second corenetwork 140 may be accessed using the LTE RAT. In another embodiment,the second core network 140 is a UMTS core network or similarthird-generation core network. Such a core network 140 may be accessedusing the LTE RAT or the UTRA RAT.

While fallback procedures are commonly used to allow interoperability ofvarious services used by the remote unit 105, conventional fallbackprocedures involve the switch from a packet-switched domain(“PS-domain”) to a circuit-switched domain (“CS-domain”). In someembodiments, a fallback procedure may involve a handover of one or morepacket-switched sessions (“PS-sessions”). In other embodiments, thefallback procedure does not handover the PS-sessions.

In mobile communication networks having a fifth-generation core network(“5GC”) and supporting a fifth-generation radio access technology, thereare many possible scenarios for mobility of a remote unit 105 betweenvarious radio access technologies and/or core networks. For example, theremote unit may switch from the 5GC via NR RAT to 5GC via LTE RAT,referred to as RAT fallback. In another example, the remote unit 105 mayswitch from the 5GC via LTE RAT to 5GC via NR RAT, referred to asinter-RAT handover. In a third example, the remote unit 105 may switchfrom the 5GC via LTE RAT to an EPC via LTE RAT, referred to as a system(or CN) fallback. In a fourth example, the remote unit 105 may switchfrom the 5GC via NR RAT to an EPC via LTE RAT, also referred to as asystem (or CN) fallback. In a fifth example, the remote unit 105 mayswitch from the 5GC via NR RAT to UMTS via UTRA, also referred to as asystem fallback. Procedures for facilitating RAT and/or CN mobility aredescribed below with reference to FIG. 3A-3B.

Previously, the fallback procedure is used to transfer the remote unit105 from 4G (EPS) to CS domain. However, when considering 5G system,fallback is possible for PS services as well, i.e., the fallback is fromPS domain to PS domain. However, when following conventional fallbackprocedures, the RAN node (e.g., base unit 110) does not have availableinformation to steer the remote unit 105 to the appropriate target RATor target system (CN).

The RRC Release procedure with redirect indication as known from LTEspecification (e.g., TS 23.272) is used to indicate to the remote unit105 the information like frequency to measure, Location Area ID (LAI),etc. However, in the 5G scenarios this information may not be sufficientin order to steer the remote unit 105 to the appropriate target RAT ortarget system (CN). Moreover, it is important for the remote unit 105 todifferentiate which target CN will be used, especially in cases of RATfallback and inter-RAT handover, in order to initiate the correct NASprocedure in the target RAT.

To overcome the above described limitations of conventional fallbackprocedures, the system 100 implements a new procedure which providescore network (CN) assisted information to the RAN node (e.g., gNB, orbase unit 110) to allow the RAN node to take appropriate decision about(1) the mobility mechanism (IDLE or CONNECTED mode mobility) and (2) thetarget RAT. In some embodiments, the CN assisted information alsoincludes the target CN for the fallback, if needed.

In particular, the first core network 130 (e.g., the AMF 131) providessupported ‘type of mobility’ and target CN (the latter in case when thetarget RAT is E-UTRAN). In various embodiments, the target CN type isalso provided to the remote unit 105 (e.g., via RRC signaling). In onealternative, the RAN node passes along the target CN information to theUE. In another alternative, the target CN type may be provided from theAMF 131 to the remote unit 105 directly using NAS signaling.

In some embodiments, when requesting a service for a possible RAT/systemfallback (e.g., emergency services), the remote unit 105 mayadditionally indicate to the first core network 130 (e.g., to the AMF131 in a NAS Service Request message) whether the remote unit 105 usesSR-mode or DR-mode of operation.

Based on 1) request from the remote unit 105 (e.g., the NAS ServiceRequest message) and/or 2) based on network configuration (e.g., whetherthe N26 interface for 5GC/EPC interworking is deployed), and/or 3) basedon the service priority/latency requirements, the AMF 131 determineswhether CONNECTED mode mobility (e.g., handover) or whether IDLE modemobility (e.g., RRC Release with redirection) is desirable.

As used herein, “IDLE mode mobility” refers to RRC Release procedurewith a redirect IE. The IDLE mode mobility may be inter-RAT IDLEmobility (e.g., within the same CN), or inter-system IDLE mobility(e.g., changing CNs). Also, as used herein, term “CONNECTED modemobility” is used to represent Handover procedure (e.g., Xn-based orN2-based or S1-based, etc.) which may be inter-RAT Handover (e.g.,within the same CN), or inter-system Handover (e.g., changing CNs).

In response to determining the mobility mode, the AMF 131 indicates inthe N2 signaling message to the RAN node (e.g., the base unit 110) atleast the following: target CNs (or a list of target CNs, includingpriorities), whether HO or IDLE mode mobility to be used, and optionallytarget RATs (e.g., a list, including priorities for different RATs).

The base unit 110 transmits the target CNs (or a list of target CNs,including priorities) to the remote unit 105. In an alternateembodiment, the AMF 131 may indicate to the remote unit 105 the targetCNs in a NAS Service Accept message to the remote unit 105.

The RAN node (e.g., base unit 110) makes the final decision about thetarget RAT of the fallback procedure considering the radio topology,radio conditions of the remote unit 105, and the indications receivedfrom the CN (e.g., from AMF 131).

The RAN node (e.g., base unit 110) needs to indicate to the remote unit105 in the RRC release message with redirection IE the target RAT andthe target CN when it decides that the radio handover is not to beperformed since it cannot be performed (e.g., when there is no handoverpreparation with the target cell possible) or when it is inefficient(will take longer time than acceptable for some latency criticalservices). It should be noted that in some cases an RRC connectionrelease with redirection may be faster than a handover procedure,especially if the redirection can be triggered blindly i.e. based oncoverage information rather than the UE measurement reports.

Based on the information received by the remote unit 105 (e.g.,information like target CN type and/or target RAT cell identifier,frequency etc. either in NAS message or in RRC message) the remote unit105 initiates NAS procedure using the NAS protocol according to thetarget CN indication and initiates the Access Stratum procedure toaccomplish the radio mobility in the target cell.

FIG. 2 depicts a first fallback procedure 200, according to embodimentsof the disclosure. The first fallback procedure 200 involves a UE 205, aRAN node 210, the AMF 131, the SMF 133, the UPF 135, and a second domain215 of the PLMN. Here, the UE 205 falls back to the second domain 215.In one embodiment, the second domain includes an IMS. In anotherembodiment, the second domain includes a circuit-switched network. Thesecond domain 215 is located in a different core network than the AMF131, SMF 133, and UPF 135 (though located in the same PLMN).

A key aspects of the first fallback procedure 200 is that the servingcore network (e.g., containing the AMF 131) provides assistanceinformation to the RAN node 210 to allow the RAN NODE 210 to make theappropriate decision about the mobility mechanism (e.g., idle modemobility vs connected mode mobility) and the target radio accesstechnology (“RAT”) for the fallback procedure. The assistanceinformation includes a target CN and, optionally, target cellinformation indicating at least one RAT. Target cell information mayinclude frequency band, RAT, and the like.

The UE 205 may be one embodiment of the remote unit 105, discussedabove. The RAN node 210 may be one embodiment of the base unit 110,discussed above. In some embodiments, the RAN node 210 is a gNB or aneNB. The second domain 215 may be one embodiment of the second corenetwork 140 and/or IMS 150.

The first fallback procedure 200 begins at Step 0, where UE 205determines to initiate a service (MO Service Initiation) (see block220). In one embodiment, the service to initiate is an emergencyservice.

In Step 1, the UE 205 sends NAS Service Request to the AMF 131 (seemessaging 225). The UE 205 includes in the NAS message at least thefollowing information: UE ID, requested type of service (e.g.,Emergency, Voice, Voice over PS (e.g., Voice over IMS) or Voice overCS), request for fallback/move to another target system. In certainembodiments, the NAS Service Request message also indicates SR/DR-modeof operation of the UE 205.

Optionally, the UE 205 may indicate radio capability to the AMF 131,e.g., whether the UE 205 is single transmission capable, or dualtransmission capable. Based on this UE 205 radio capability, the AMF 131determines whether a CONNECTED mode (e.g., Handover procedure) or IDLEmode mobility (e.g., RRC Release with redirection) is desirable. Notethat the term IDLE mode mobility is used to represent RRC Releaseprocedure with redirect IE.

The ‘request for fallback/move to another target system’ may for exampleindicate to the network (e.g., AMF 131 or SMF 133) which would be thetarget system, e.g., EPS (if the UE 205 is currently connected to 5GS)or 5GS (if the UE 205 is currently connected to EPS).

In Step 2, the AMF 131 may determine the target RAT(s) (e.g., E-UTRAN,UTRAN, GERAN) and target CN(s) (e.g., in case of target RAT is E-UTRANthe target CN may be EPC or 5GC) which would support the requestedservice (see block 230). In addition, the AMF 131 considers the networkdeployment (e.g., whether N26 interface is supported) in order todetermine whether CONNECTED mode mobility can be supported, or whetherIDLE mode mobility is preferable. The AMF 131 indicates this informationto the RAN node 210 as “type of mobility” indication. For example, iffallback to EPC is required and N26 interface is supported, thenCONNECTED mode mobility is preferable. However, if fallback to EPC isrequired and N26 is not supported, then AMF 131 may indicate that IDLEmode mobility is preferable.

In one embodiment, the target CN is indicated to RAN node 210 or UE 205only if there may be ambiguity at the UE 205. For example, if the targetRAT may be connected to multiple CNs, then the target CN is indicated tothe RAN/UE. In a particular example, if the target RAT is E-UTRAN (orknown as LTE) and LTE cell is connected to both EPC and 5GC, thenredirecting the UE 205 in IDLE state to the LTE cell would causeambiguity. One proposal is that the AMF 131 decide to indicate target CNtype to RAN node 210 or UE 205 (in step 3 below) only if there isambiguity of the CN to be used. In another example, if the target RAT isUTRAN (e.g., 3G), then there is no ambiguity for the target CN, as theCN type is GPRS.

As the AMF 131 usually does not know the exact coverage conditions ofthe potential target RAT(s), the AMF 131 may create a list of targetRATs/CNs based on preference or order of selection. This kind ofpreference list would steer the RAN node 210 (e.g., gNB) to takedecision about the target RAT based on the coverage condition at the UE205 location (e.g., depending on the radio measurements reported by theUE 205 to the source RAN node 210).

Further, if the UE 205 is in CONNECTED state, the AMF 131 may considerthe service priority/latency requirements of the PDU Session used by theUE 205 when creating the fallback information to be sent to the RAN node210. Based on this, the AMF 131 may send to the RAN node 210 informationabout the criticality (e.g., priority or latency requirements) of thefallback procedure. For example, if the UE 205 is using a PDU Session(e.g., the UE 205 resources are activated) having a packet deliveryrequirement for latency, the AMF 131 may indicate to the RAN node 210that the mobility procedure shall meet the latency/priorityrequirements.

In Step 3 a, the AMF 131 sends N2 request signaling message to the RANnode 210 (e.g., gNB) to request a fallback procedure for the UE 205. TheAMF 131 indicates in addition at least one target CN type to which theUE 205 has to fallback. The RAN node 210 takes the target CN informationin order to select to the correct cell to which IDLE mode or CONNECTEDmode procedure needs to be performed. The AMF 131 may indicate at leastone of the following fallback information to the RAN node 210: TargetRATs, Target Tracking Area (TA), Requested service, Type of mobility,and/or Criticality requirements.

As used herein, the “Target RATs” refers to an indication of at leastone RAT to which the UE 205 has to be moved in order to perform therequested service. “Target Tracking Area (TA)” refers to informationhelps the RAN node 210 to find proper target cell as part of the TAindicated by the CN. The “Requested service” refers to the service whichthe UE 205 would like to use the and which triggered the procedure forfallback or RAT/System change. The “Type of mobility” refers to anindication of whether CONNECTED mode or IDLE mode mobility is preferredbased on the configuration in the CN. For example, if N26 interface isdeployed, the AMF 131 may indicate CONNECTED state mobility is desirableand the AMF 131 based on this may include the AS context (e.g., ASsecurity, MM context, radio capabilities, etc.) to the gNB. If, e.g.,the N26 interface is not deployed, the CN may indicate ‘IDLE modemobility’ or ‘similarly RRC Release with redirect’ or similarindication. “Criticality requirements” refers to an indication of theservice priority/latency requirements of the PDU Session currently usedby the UE 205.

The RAN node 210 decides whether to perform IDLE mode mobility (e.g., toexecute RRC release procedure with redirection IE) or to performCONNECTED mode mobility (e.g., to execute a Handover procedure) based onthe UE 205's radio coverage and on the indications received from the AMF131.

Step 3 b shows the case where the RAN node 210 decides to perform IDLEmode mobility, the RAN node 210 performs RRC release procedure and sendsan RRC Connection Release message to the UE 205 which includes target CNinformation (see messaging 235). In one embodiment, the RRC ConnectionRelease message includes redirection information, the redirectioninformation including the target CN information and, optionally, targetRAN information.

Alternatively, the RAN node 210 initiates handover procedure to thetarget RAT/CN, when this is possible, with or without configuringmeasurements to the UE 205. The latter case is Blind handover case. Inthe former case, the actual handover execution shall be delayed untilthe UE 205 performs measurements and provides the results to source RAT,the source RAT in turn prepares/informs the target RAT for the same, thetarget RAT prepared handover command and sends to the UE 205 via thesource RAT.

Step 3 c shows the case where the RAN node 210 decides to performCONNECTED mode mobility, the RAN node 210 may perform RRC ConnectionReconfiguration procedure in order to configure the UE 205 withmeasurements to the desired target cell (see messaging 240). In thefollowing a RAN node 210 initiates a Handover procedure to the targetRAT cell.

In Step 4, the RAN node 210 sends N2 request Ack message to the AMF 131(see messaging 245). If the RAN node 210 has decided to perform IDLEmode mobility, the RAN node 210 includes corresponding information tothe AMF 131 indicating that the UE 205 has been redirected to aparticular RAT. However, if the RAN node 210 has decided to performCONNECTED mode mobility, the RAN node 210 performs actions according toXn-based or N2-based Handover procedures.

Based on whether the RAN node 210 selects IDLE mode mobility orCONNECTED mode mobility, the RAN 210 performs one of the following:

Step 5 a: If the RAN node 210 has performed IDLE mode mobility, the AMF131 determines whether to release the existing PDU Sessions, or topreserve the PDU Session context in the CN but deactivate the UPresources (in case the UE 205 has been in CONNECTED state and the UPresources have been activated; see block 250). For example, the AMF 131may initiate N11 exchange towards the SMF 133 to release the PDU SessionUP resources. If the AMF 131 decides to preserve the PDU Sessionscontext in the SGC, the AMF 131 may also indicate to the SMF 133 thatthe PDU Session is temporary suspended in order to avoid the SMF 133initiating paging procedure.

Step 5 b: If the RAN node 210 has performed CONNECTED mode mobility(e.g., a Handover procedure), then the AMF 131 continues with theHandover procedure (see block 255).

In Step 6, the UE 205 initiates NAS procedure in the target RAT usingthe NAS protocol according to the target CN indication (e.g., EPC NAS or5GC NAS) received in the RRC message (see block 260). In certainembodiments, the RRC message may indicate to the UE 205 that theredirection may be performed to a particular CN Type (e.g., 5GS or EPS)and in the target RAT cell. In such embodiments, the UE 205 uses thecorresponding NAS protocol accordingly.

Moreover, subsequent to the radio procedure to move the UE 205 in thetarget RAT cell using, e.g., RRC Idle or RRC INACTIVE state cellredirection principles, the NAS protocol corresponding to the indicatedCN Type shall be used. An ASN.1 encoding of including the CN Type (e.g.,5GS or EPS) in the RRC message releasing the RRC Connection is shownFIG. 4.

In Step 7, the UE 205 imitates a service call in the target RAT and/ortarget CN (“target system”), depicted here as the second domain 215 (seeblock 265). If IDLE mode mobility was performed and after the UE 205completes the RRC procedure establishment in the target cell andperforms NAS registration (or attach) with the target CN, the UE 205initiates the requested service (see block 270). Alternatively, therequested service may be also indicated during the NAS registration (orattach) procedure. The first fallback procedure 200 ends.

FIG. 3 depicts a second fallback procedure 300, according to embodimentsof the disclosure. The second fallback procedure 300 involves the UE205, the RAN node 210, the AMF 131, the SMF 133, the UPF 135, and thesecond domain 215 of the PLMN. Here, the UE 205 falls back to the seconddomain 215.

A key aspects of the second fallback procedure 300 is that the servingcore network (e.g., containing the AMF 131) provides assistanceinformation to the RAN node 210 to allow the RAN NODE 210 to make theappropriate decision about the mobility mechanism (e.g., idle modemobility vs connected mode mobility) and the target radio accesstechnology (“RAT”) for the fallback procedure. The assistanceinformation includes a target CN and, optionally, target cellinformation indicating at least one RAT. Target cell information mayinclude frequency band, RAT, and the like.

The second fallback procedure 300 begins at Step 0, where UE 205determines to initiate a service (MO Service Initiation) (see block305). In one embodiment, the service to initiate is an emergencyservice.

In Step 1, the UE 205 sends NAS Service Request to the AMF 131 (seemessaging 310). The UE 205 includes in the NAS message at least thefollowing information: UE ID, requested type of service (e.g.,Emergency, Voice, Voice over PS (e.g., Voice over IMS) or Voice overCS), request for fallback/move to another target system. In certainembodiments, the NAS Service Request message also indicates a SR/DR-modeof operation of the UE 205.

Optionally, the UE 205 may indicate radio capability to the AMF 131,e.g., whether the UE 205 is single transmission capable, or dualtransmission capable. Based on this UE 205 radio capability, the AMF 131may determine whether a CONNECTED mode (e.g., Handover procedure) orIDLE mode mobility (e.g., RRC Release with redirection) is desirable.Note that the term IDLE mode mobility is used to represent RRC Releaseprocedure with redirect IE.

The ‘request for fallback/move to another target system’ may indicate tothe network (e.g., AMF 131 or SMF 133) which would be the target system,e.g., EPS (if the UE 205 is currently connected to 5GS) or 5GS (if theUE 205 is currently connected to EPS).

In Step 2, the AMF 131 may determine the target RAT(s) (e.g., E-UTRAN,UTRAN, GERAN) and target CN(s) (e.g., in case of target RAT is E-UTRANthe target CN may be EPC or 5GC) which would support the requestedservice (see block 315). In addition, the AMF 131 considers the networkdeployment (e.g., whether N26 interface is supported) in order todetermine whether CONNECTED mode mobility can be supported, or whetherIDLE mode mobility is preferable. The AMF 131 indicates this informationto the RAN node 210 as “type of mobility” indication. For example, iffallback to EPC is required and N26 interface is supported, thenCONNECTED mode mobility is preferable. However, if fallback to EPC isrequired and N26 is not supported, then AMF 131 may indicate that IDLEmode mobility is preferable.

In Step 3 a, the AMF 131 sends a NAS Service Request message to the UE205 containing a proper reject cause value (see messaging 320). Forexample, the reject cause may indicate that the requested service is notsupported. The AMF 131 may also indicate a list of possible RATs/CNs inwhich the requested service may be used. If the AMF 131 has detectedthat the target RAT may be connected to multiple CNs (e.g., in case ofE-UTRA), the AMF 131 may include a target CN indication to the UE 205.The target CN points to the UE 205 which NAS protocols stack to be usedafter the fallback procedure.

In Step 3 b, the AMF 131 sends a N2 request signaling message to the RANnode 210 (e.g., gNB) to request a fallback procedure for the UE 205 (seemessaging 325). The AMF 131 may indicate at least one of the followingfallback information to the RAN node 210: Target RATs, Target TrackingArea (TA), Requested service, Type of mobility, and/or Criticalityrequirements. Note that the NAS Service Request message from Step 3 amay be sent encapsulated in the same N2 message as Step 3 b. The RANnode 210 should correspondingly process and transfer first theencapsulated NAS message to the UE 205 before releasing the RRCconnection.

The RAN node 210 decides whether to perform IDLE mode mobility (e.g., toexecute RRC release procedure with redirection IE) or to performCONNECTED mode mobility (e.g., to execute a Handover procedure) based onthe UE 205's radio coverage and on the indications received from the AMF131.

Step 3 c shows the case where the RAN node 210 decides to perform IDLEmode mobility, the RAN node 210 performs RRC release procedure (seemessaging 330). Alternatively, the RAN node 210 initiates handoverprocedure to the target RAT/CN, when this is possible, with or withoutconfiguring measurements to the UE 205. The latter case is Blindhandover case. In the former case, the actual handover execution shallbe delayed until the UE 205 performs measurements and provides theresults to source RAT, the source RAT in turn prepares/informs thetarget RAT for the same, the target RAT prepared handover command andsends to the UE 205 via the source RAT.

Step 3 d shows the case where the RAN node 210 decides to performCONNECTED mode mobility, the RAN node 210 may perform RRC ConnectionReconfiguration procedure in order to configure the UE 205 withmeasurements to the desired target cell (see messaging 335). In thefollowing a RAN node 210 initiates a Handover procedure to the targetRAT cell.

In Step 4, the RAN node 210 sends N2 request Ack message to the AMF 131(see messaging 340). If the RAN node 210 has decided to perform IDLEmode mobility, the RAN node 210 includes corresponding information tothe AMF 131 indicating that the UE 205 has been redirected to aparticular RAT. However, if the RAN node 210 has decided to performCONNECTED mode mobility, the RAN node 210 performs actions according toXn-based or N2-based Handover procedures.

Based on whether the RAN node 210 selects IDLE mode mobility orCONNECTED mode mobility, the RAN 210 performs one of the following:

Step 5 a: If the RAN node 210 has performed IDLE mode mobility, the AMF131 determines whether to release the existing PDU Sessions, or topreserve the PDU Session context in the CN but deactivate the UPresources (in case the UE 205 has been in CONNECTED state and the UPresources have been activated; see block 345). For example, the AMF 131initiates N11 exchange towards the SMF 133 to release the PDU Session UPresources. If the AMF 131 decides to preserve the PDU Sessions contextin the SGC, the AMF 131 may also indicate to the SMF 133 that the PDUSession is temporary suspended in order to avoid the SMF 133 to initiatepaging procedure.

Step 5 b: If the RAN node 210 has performed CONNECTED mode mobility(e.g., a Handover procedure), the AMF 131 continues with the Handoverprocedure (see block 350).

In Step 6, the UE 205 initiates NAS procedure in the target RAT usingthe NAS protocol according to the target CN indication (e.g., EPC NAS or5GC NAS) received in the NAS message (see block 355). In one possibleoption, the NAS message may indicate to the UE 205 that the redirectionmay be performed to a particular CN Type (e.g., 5GS or EPS) and in thetarget RAT cell, the UE 205 shall use the corresponding NAS protocolaccordingly.

In Step 7, the UE 205 imitates a service call in the target RAT and/ortarget CN (“target system”), depicted here is the second domain 215 (seeblock 360). If IDLE mode mobility was performed and after the UE 205completes the RRC procedure establishment in the target cell andperforms NAS registration (or attach) with the target CN, the UE 205initiates the requested service. Alternatively, the requested servicemay be also indicated during the NAS registration (or attach) procedure.

FIG. 4 depicts an RRC connection message 400 according to embodiments ofthe disclosure. Elements/parameters to communicate the CN assistedinformation, e.g., for use in the above described fallback procedures,are shown in bold and italics.

FIG. 5 depicts one embodiment of a user equipment apparatus 500 that maybe used for providing fallback assistance information to a RAN node. Theuser equipment apparatus 500 may be one embodiment of the remote unit105. Furthermore, the user equipment apparatus 500 may include aprocessor 505, a memory 510, an input device 515, an output device 520,and a transceiver 525. In some embodiments, the input device 515 and theoutput device 520 are combined into a single device, such as atouchscreen. In certain embodiments, the user equipment apparatus 500may not include any input device 515 and/or output device 520. Invarious embodiments, the user equipment apparatus 500 may include one ormore of: the processor 505, the memory 510, and the transceiver 525, andmay not include the input device 515 and/or the output device 520.

The processor 505, in one embodiment, may include any known controllercapable of executing computer-readable instructions and/or capable ofperforming logical operations. For example, the processor 505 may be amicrocontroller, a microprocessor, a central processing unit (“CPU”), agraphics processing unit (“GPU”), an auxiliary processing unit, a fieldprogrammable gate array (“FPGA”), or similar programmable controller. Insome embodiments, the processor 505 executes instructions stored in thememory 510 to perform the methods and routines described herein. Theprocessor 505 is communicatively coupled to the memory 510, the inputdevice 515, the output device 520, and the transceiver 525.

In some embodiments, the transceiver 525 sends a service request andreceives a connection release message. Here, the service requestrequires fallback to at least one of: a different RAT and a differentCN. Moreover, the connection release message includes redirectioninformation for the service fallback, the redirection informationincluding a target CN. The processor 505 that selects a NAS procedurebased on the target CN and connects to the target CN using the selectedNAS procedure.

In some embodiments, the NAS procedure in the target CN is one of: anEPC NAS procedure and a 5GC NAS procedure. In various embodiments, theNAS procedure in the target CN includes initiating the requested servicevia the target CN. In certain embodiments, sending the service requestincludes indicating at least one of: type of the requested service and aradio capability of the remote unit, the radio capability including anindication of whether the remote unit is dual-transmission capable.

In some embodiments, the redirection information further includes atleast one target cell indicating at least one RAT. In certainembodiments, the indicated RAT is E-UTRAN, wherein the processor furtherprovides the target CN to one or more upper layers.

The memory 510, in one embodiment, is a computer readable storagemedium. In some embodiments, the memory 510 includes volatile computerstorage media. For example, the memory 510 may include a RAM, includingdynamic RAM (“DRAM”), synchronous dynamic RAM (“SDRAM”), and/or staticRAM (“SRAM”). In some embodiments, the memory 510 includes non-volatilecomputer storage media. For example, the memory 510 may include a harddisk drive, a flash memory, or any other suitable non-volatile computerstorage device. In some embodiments, the memory 510 includes bothvolatile and non-volatile computer storage media.

In some embodiments, the memory 510 stores data related to providingfallback assistance information to a RAN node. For example, the memory510 may store target CN information, target RAT information, and thelike. In certain embodiments, the memory 510 also stores program codeand related data, such as an operating system or other controlleralgorithms operating on the remote unit 105.

The input device 515, in one embodiment, may include any known computerinput device including a touch panel, a button, a keyboard, a stylus, amicrophone, or the like. In some embodiments, the input device 515 maybe integrated with the output device 520, for example, as a touchscreenor similar touch-sensitive display. In some embodiments, the inputdevice 515 includes a touchscreen such that text may be input using avirtual keyboard displayed on the touchscreen and/or by handwriting onthe touchscreen. In some embodiments, the input device 515 includes twoor more different devices, such as a keyboard and a touch panel.

The output device 520, in one embodiment, is designed to output visual,audible, and/or haptic signals. In some embodiments, the output device520 includes an electronically controllable display or display devicecapable of outputting visual data to a user. For example, the outputdevice 520 may include, but is not limited to, an LCD display, an LEDdisplay, an OLED display, a projector, or similar display device capableof outputting images, text, or the like to a user. As another,non-limiting, example, the output device 520 may include a wearabledisplay separate from, but communicatively coupled to, the rest of theuser equipment apparatus 500, such as a smart watch, smart glasses, aheads-up display, or the like. Further, the output device 520 may be acomponent of a smart phone, a personal digital assistant, a television,a table computer, a notebook (laptop) computer, a personal computer, avehicle dashboard, or the like.

In certain embodiments, the output device 520 includes one or morespeakers for producing sound. For example, the output device 520 mayproduce an audible alert or notification (e.g., a beep or chime). Insome embodiments, the output device 520 includes one or more hapticdevices for producing vibrations, motion, or other haptic feedback. Insome embodiments, all or portions of the output device 520 may beintegrated with the input device 515. For example, the input device 515and output device 520 may form a touchscreen or similar touch-sensitivedisplay. In other embodiments, the output device 520 may be located nearthe input device 515.

The transceiver 525 includes at least transmitter 530 and at least onereceiver 535. One or more transmitters 530 may be used to communicatewith a RAN node, such as the base unit 110, Although only onetransmitter 530 and one receiver 535 are illustrated, the user equipmentapparatus 500 may have any suitable number of transmitters 530 andreceivers 535. Moreover, the transceiver 525 may support one or morenetwork interfaces 540. For example, the transceiver 525 may support aUu interface for communication with a RAN node, an N1 interface forcommunication with an AMF, and the like.

FIG. 6 depicts one embodiment of a RAN apparatus 600 that may be usedfor providing fallback assistance information to a RAN node. The RANapparatus 600 may be one embodiment of the base unit 110 and/or RAN node210. Furthermore, the RAN apparatus 600 may include a processor 605, amemory 610, an input device 615, an output device 620, and a transceiver625. In some embodiments, the input device 615 and the output device 620are combined into a single device, such as a touchscreen. In certainembodiments, the RAN apparatus 600 may not include any input device 615and/or output device 620. In various embodiments, the RAN apparatus 600may include one or more of: the processor 605, the memory 610, and thetransceiver 625, and may not include the input device 615 and/or theoutput device 620.

The processor 605, in one embodiment, may include any known controllercapable of executing computer-readable instructions and/or capable ofperforming logical operations. For example, the processor 605 may be amicrocontroller, a microprocessor, a central processing unit (“CPU”), agraphics processing unit (“GPU”), an auxiliary processing unit, a fieldprogrammable gate array (“FPGA”), or similar programmable controller. Insome embodiments, the processor 605 executes instructions stored in thememory 610 to perform the methods and routines described herein. Theprocessor 605 is communicatively coupled to the memory 610, the inputdevice 615, the output device 620, and the transceiver 625.

In various embodiments, the transceiver 625 receives a first messagefrom a network function in a first core network, wherein the firstmessage indicates service fallback of a remote unit connected to the RANnode and indicates a target CN. The processor 605 defines and/orconfigures service fallback parameters for the remote unit. Moreover,the transceiver 625 sends a connection release message to the remoteunit, the connection release message including redirection informationfor the service fallback, the redirection information including thetarget CN.

In some embodiments, the redirection information further includes atarget radio access technology RAT. In certain embodiments, the firstmessage additionally includes RAN node information, wherein the RAN nodeinformation comprises at least one of: UE security context and UEmobility restrictions.

In some embodiments, the first message includes a target RAT. In suchembodiments, defining and/or configuring the service fallback parametersincludes selecting a fallback RAT based on the target RAT and one ormore of: radio topology, and radio conditions of the remote unit.

In some embodiments, the first message includes target fallbackinformation, the target fallback information including one or more of: atarget radio access technology (“RAT”), the target CN, a target mobilitytype, a service request by the remote unit that triggered the servicefallback, and criticality requirements of a data connection of theremote unit.

The memory 610, in one embodiment, is a computer readable storagemedium. In some embodiments, the memory 610 includes volatile computerstorage media. For example, the memory 610 may include a RAM, includingdynamic RAM (“DRAM”), synchronous dynamic RAM (“SDRAM”), and/or staticRAM (“SRAM”). In some embodiments, the memory 610 includes non-volatilecomputer storage media. For example, the memory 610 may include a harddisk drive, a flash memory, or any other suitable non-volatile computerstorage device. In some embodiments, the memory 610 includes bothvolatile and non-volatile computer storage media.

In some embodiments, the memory 610 stores data related to providingfallback assistance information to a RAN node. For example, the memory610 may store target CN information, target RAT information, and thelike. In certain embodiments, the memory 610 also stores program codeand related data, such as an operating system or other controlleralgorithms operating on the remote unit 105.

The input device 615, in one embodiment, may include any known computerinput device including a touch panel, a button, a keyboard, a stylus, amicrophone, or the like. In some embodiments, the input device 615 maybe integrated with the output device 620, for example, as a touchscreenor similar touch-sensitive display. In some embodiments, the inputdevice 615 includes a touchscreen such that text may be input using avirtual keyboard displayed on the touchscreen and/or by handwriting onthe touchscreen. In some embodiments, the input device 615 includes twoor more different devices, such as a keyboard and a touch panel.

The output device 620, in one embodiment, is designed to output visual,audible, and/or haptic signals. In some embodiments, the output device620 includes an electronically controllable display or display devicecapable of outputting visual data to a user. For example, the outputdevice 620 may include, but is not limited to, an LCD display, an LEDdisplay, an OLED display, a projector, or similar display device capableof outputting images, text, or the like to a user. As another,non-limiting, example, the output device 620 may include a wearabledisplay separate from, but communicatively coupled to, the rest of theRAN apparatus 600, such as a smart watch, smart glasses, a heads-updisplay, or the like. Further, the output device 620 may be a componentof a smart phone, a personal digital assistant, a television, a tablecomputer, a notebook (laptop) computer, a personal computer, a vehicledashboard, or the like.

In certain embodiments, the output device 620 includes one or morespeakers for producing sound. For example, the output device 620 mayproduce an audible alert or notification (e.g., a beep or chime). Insome embodiments, the output device 620 includes one or more hapticdevices for producing vibrations, motion, or other haptic feedback. Insome embodiments, all or portions of the output device 620 may beintegrated with the input device 615. For example, the input device 615and output device 620 may form a touchscreen or similar touch-sensitivedisplay. In other embodiments, the output device 620 may be located nearthe input device 615.

The transceiver 625 includes at least transmitter 630 and at least onereceiver 635. One or more transmitters 630 may be used to provide DLcommunication signals to a remote unit 105. Similarly, one or morereceivers 635 may be used to receive UL communication signals from theremote unit 105, as described herein. Although only one transmitter 630and one receiver 635 are illustrated, the RAN apparatus 600 may have anysuitable number of transmitters 630 and receivers 635. Further, thetransmitter(s) 625 and the receiver(s) 630 may be any suitable type oftransmitters and receivers. In one embodiment, the transceiver 625includes transmitter/receiver pair(s) to communicate with a mobilecommunication network, including the first core network 130 and secondcore network 140. Moreover, the transceiver 625 may support one or morenetwork interfaces 640. For example, the transceiver 625 may support aUu interface for communication with a UE, an N2 interface forcommunication with an AMF, and the like.

FIG. 7 depicts one embodiment of a network function apparatus 700 thatmay be used for providing fallback assistance information to a RAN node.The network function apparatus 700 may be one embodiment of the remoteunit 105. Furthermore, the network function apparatus 700 may include aprocessor 705, a memory 710, an input device 715, an output device 720,and a transceiver 725. In some embodiments, the input device 715 and theoutput device 720 are combined into a single device, such as atouchscreen. In certain embodiments, the network function apparatus 700may not include any input device 715 and/or output device 720. Invarious embodiments, the network function apparatus 700 may include oneor more of: the processor 705, the memory 710, and the transceiver 725,and may not include the input device 715 and/or the output device 720.

The processor 705, in one embodiment, may include any known controllercapable of executing computer-readable instructions and/or capable ofperforming logical operations. For example, the processor 705 may be amicrocontroller, a microprocessor, a central processing unit (“CPU”), agraphics processing unit (“GPU”), an auxiliary processing unit, a fieldprogrammable gate array (“FPGA”), or similar programmable controller. Insome embodiments, the processor 705 executes instructions stored in thememory 710 to perform the methods and routines described herein. Theprocessor 705 is communicatively coupled to the memory 710, the inputdevice 715, the output device 720, and the transceiver 725.

In some embodiments, the transceiver 725 receives a service request froma remote unit, wherein the service request requires fallback to at leastone of: a different RAT and a different CN. The processor 705 identifiesat least one of: a target RAT and a target CN, based on at least one of:the requested service, remote unit capabilities, and networkconfiguration. Moreover, the transceiver 725 indicates, to a RAN node,the at least one of: a target RAT and a target CN, based on therequested service, wherein the RAN node performs fallback procedure withthe remote unit based on the at least one of: a target RAT and a targetCN. Here, performing fallback procedure includes one of:

In some embodiments, the service request indicates that the remote unitrequires emergency services fallback. In certain embodiments, indicatingthe at least one of: a target RAT and a target CN, includes sending atleast one of: a prioritized list of target RATs and a prioritized listof CNs.

In some embodiments, the transceiver 725 further sends a signalingmessage with fallback information to the RAN node, wherein the fallbackinformation includes at least one of: a target RAT, a target trackingarea, the request service, a mobility type, and service requirements ofa data connection used by the remote unit. In such embodiments, themobility type indicates idle mode mobility of the remote unit, thetransceiver receives an acknowledgment message from the RAN node, andthe processor, in response to the acknowledgment message, performs oneof: releasing a data connection used by the remote unit, suspending adata connection used by the remote unit, and handing over a dataconnection used by the remote unit.

The memory 710, in one embodiment, is a computer readable storagemedium. In some embodiments, the memory 710 includes volatile computerstorage media. For example, the memory 710 may include a RAM, includingdynamic RAM (“DRAM”), synchronous dynamic RAM (“SDRAM”), and/or staticRAM (“SRAM”). In some embodiments, the memory 710 includes non-volatilecomputer storage media. For example, the memory 710 may include a harddisk drive, a flash memory, or any other suitable non-volatile computerstorage device. In some embodiments, the memory 710 includes bothvolatile and non-volatile computer storage media.

In some embodiments, the memory 710 stores data related to providingfallback assistance information to a RAN node. For example, the memory710 may store target CN information, target RAT information, and thelike. In certain embodiments, the memory 710 also stores program codeand related data, such as an operating system or other controlleralgorithms operating on the remote unit 105.

The input device 715, in one embodiment, may include any known computerinput device including a touch panel, a button, a keyboard, a stylus, amicrophone, or the like. In some embodiments, the input device 715 maybe integrated with the output device 720, for example, as a touchscreenor similar touch-sensitive display. In some embodiments, the inputdevice 715 includes a touchscreen such that text may be input using avirtual keyboard displayed on the touchscreen and/or by handwriting onthe touchscreen. In some embodiments, the input device 715 includes twoor more different devices, such as a keyboard and a touch panel.

The output device 720, in one embodiment, is designed to output visual,audible, and/or haptic signals. In some embodiments, the output device720 includes an electronically controllable display or display devicecapable of outputting visual data to a user. For example, the outputdevice 720 may include, but is not limited to, an LCD display, an LEDdisplay, an OLED display, a projector, or similar display device capableof outputting images, text, or the like to a user. As another,non-limiting, example, the output device 720 may include a wearabledisplay separate from, but communicatively coupled to, the rest of thenetwork function apparatus 700, such as a smart watch, smart glasses, aheads-up display, or the like. Further, the output device 720 may be acomponent of a smart phone, a personal digital assistant, a television,a table computer, a notebook (laptop) computer, a personal computer, avehicle dashboard, or the like.

In certain embodiments, the output device 720 includes one or morespeakers for producing sound. For example, the output device 720 mayproduce an audible alert or notification (e.g., a beep or chime). Insome embodiments, the output device 720 includes one or more hapticdevices for producing vibrations, motion, or other haptic feedback. Insome embodiments, all or portions of the output device 720 may beintegrated with the input device 715. For example, the input device 715and output device 720 may form a touchscreen or similar touch-sensitivedisplay. In other embodiments, the output device 720 may be located nearthe input device 715.

The transceiver 725 includes at least transmitter 730 and at least onereceiver 735. One or more transmitters 730 and one or more receivers 735may be used to communicate with a base unit 110, such as the RAN node210, and/or with other network functions in a core network. Althoughonly one transmitter 730 and one receiver 735 are illustrated, thenetwork function apparatus 700 may have any suitable number oftransmitters 730 and receivers 735. Further, the transmitter(s) 725 andthe receiver(s) 730 may be any suitable type of transmitters andreceivers. Moreover, the transceiver 725 may support one or more networkinterfaces 740. For example, the transceiver 725 may support an N1interface for communication with a UE, an N11 interface forcommunication with a SMF, and the like.

FIG. 8 depicts a method 800 for providing fallback assistanceinformation to a RAN node, according to embodiments of the disclosure.In some embodiments, the method 1000 is performed by an apparatus, suchas the remote unit 105, the UE 205, and/or the user equipment apparatus500. In certain embodiments, the method 800 may be performed by aprocessor executing program code, for example, a microcontroller, amicroprocessor, a CPU, a GPU, an auxiliary processing unit, a FPGA, orthe like.

The method 800 begins and sends 805, by a remote unit, a servicerequest, wherein the service request requires fallback to at least oneof: a different RAT and a different CN. In certain embodiments, sending805 the service request includes indicating at least one of: type of therequested service and a radio capability of the remote unit, the radiocapability including an indication of whether the remote unit isdual-transmission capable.

The method 800 includes receiving 810, at the remote unit, a connectionrelease message, where the connection release message includesredirection information for the service fallback, the redirectioninformation including a target CN. In some embodiments, the redirectioninformation further includes target cell information indicating at leastone RAT. In certain embodiments, the indicated RAT is E-UTRAN.

The method 800 includes selecting 815 a NAS procedure based on thetarget CN.

The method 800 includes connecting 820 to the target CN using theselected NAS procedure. The method 800 ends. In some embodiments, theNAS procedure in the target CN may be one of: an EPC NAS procedure and a5GC NAS procedure. In various embodiment, the NAS procedure in thetarget CN may include initiating the requested service via the targetCN.

FIG. 9 depicts a method 900 for providing fallback assistanceinformation to a RAN node, according to embodiments of the disclosure.In some embodiments, the method 1000 is performed by a RAN nodeapparatus, such as the base unit 110, the RAN node 210, and/or the RANapparatus 600. In certain embodiments, the method 900 may be performedby a processor executing program code, for example, a microcontroller, amicroprocessor, a CPU, a GPU, an auxiliary processing unit, a FPGA, orthe like.

The method 900 begins and receives 905 a first message from a networkfunction in a first core network, wherein the first message indicatesservice fallback of a remote unit connected to the RAN node andindicates a target CN. In certain embodiments, the first messageadditionally includes RAN node information, wherein the RAN nodeinformation includes at least one of: UE security context and UEmobility restrictions. In certain embodiments, the first messageincludes target fallback information, the target fallback informationincluding one or more of: a target RAT, the target CN, a target mobilitytype, a service request by the remote unit that triggered the servicefallback, and criticality requirements of a data connection of theremote unit.

The method 900 includes defining 910 service fallback parameters for theremote unit. Here, the target CN is used to execute either inter-RATredirection or inter-system redirection, where inter-system redirectionis triggered if the target CN is different than a current CN of theremote unit. In certain embodiments, defining the service fallbackparameters includes selecting a fallback RAT based on the target RAT andone or more of: radio topology, and radio conditions of the remote unit.

The method 900 includes sending 915 a connection release message to theremote unit, where the connection release message includes redirectioninformation for the service fallback. Here, the redirection informationincludes a target RAT and the target CN. In some embodiments, the targetCN is used to resolve the target RAT cell. The method 900 ends.

FIG. 10 depicts a method 1000 for providing fallback assistanceinformation to a RAN node, according to embodiments of the disclosure.In some embodiments, the method 1000 is performed by an apparatus, suchas the AMF 131 and/or the network function apparatus 700. In certainembodiments, the method 1000 may be performed by a processor executingprogram code, for example, a microcontroller, a microprocessor, a CPU, aGPU, an auxiliary processing unit, a FPGA, or the like.

The method 1000 begins and receives 1005, at a network function part ofa first core network, a service request from a remote unit, wherein theservice request is for a service which requires fallback to at least oneof: a different RAT and a different CN. In some embodiments, the servicerequest indicates that the remote unit requires emergency servicesfallback.

The method 1000 includes identifying 1010, at the network function, atleast one of: a target RAT and a target CN, based on at least one of:the requested service, remote unit capabilities and networkconfiguration.

The method 1000 includes indicating 1015, to a RAN node, the at leastone of: a target RAT and a target CN, based on the requested service,wherein the RAN node performs fallback with the remote unit based on theat least one of: a target RAT and a target CN. In certain embodiments,indicating the at least one of: a target RAT and a target CN, includingsending at least one of: a prioritized list of target RATs and aprioritized list of CNs. In some embodiments, the network function sendsa signaling message with fallback information to the RAN node, whereinthe fallback information includes at least one of: a target RAT, atarget tracking area, the request service, a mobility type, and servicerequirements of a data connection used by the remote unit.

Disclosed herein is a first apparatus for providing fallback assistanceinformation. The first apparatus may be one embodiment of the remoteunit 105, the UE 205, and/or the user equipment apparatus 500. The firstapparatus includes a transceiver that sends a service request andreceives a connection release message. Here, the service requestrequires fallback to at least one of: a different RAT and a differentCN. Moreover, the connection release message includes redirectioninformation for the service fallback, the redirection informationincluding a target CN. The first apparatus includes a processor thatselects a NAS procedure based on the target CN and connects to thetarget CN using the selected NAS procedure.

In some embodiments, the NAS procedure in the target CN is one of: anEPC NAS procedure and a 5GC NAS procedure. In various embodiments, theNAS procedure in the target CN includes initiating the requested servicevia the target CN. In certain embodiments, sending the service requestincludes indicating at least one of: type of the requested service and aradio capability of the remote unit, the radio capability including anindication of whether the remote unit is dual-transmission capable.

In some embodiments, the redirection information further includes atleast one target cell indicating at least one RAT. In certainembodiments, the indicated RAT is E-UTRAN, wherein the processor furtherprovides the target CN to one or more upper layers.

Disclosed herein is a first method for providing fallback assistanceinformation. The first method may be implemented by the remote unit 105,the UE 205, and/or the user equipment apparatus 500. The first methodincludes sending, by a remote unit, a service request, wherein theservice request requires fallback to at least one of: a different RATand a different CN. The first method includes receiving, at the remoteunit, a connection release message, where the connection release messageincludes redirection information for the service fallback, theredirection information including a target CN. The first method includesselecting a NAS procedure based on the target CN. The first methodincludes connecting to the target CN using the selected NAS procedure.

In some embodiments, the NAS procedure in the target CN may be one of:an EPC NAS procedure and a 5GC NAS procedure. In various embodiment, theNAS procedure in the target CN may include initiating the requestedservice via the target CN. In certain embodiments, sending the servicerequest includes indicating at least one of: type of the requestedservice and a radio capability of the remote unit, the radio capabilityincluding an indication of whether the remote unit is dual-transmissioncapable.

In some embodiments, the redirection information further includes targetcell information indicating at least one RAT. In certain embodiments,the indicated RAT is E-UTRAN. In such embodiments, the first methodfurther includes providing the target CN to one or more upper layers inthe remote unit.

Disclosed herein is a second apparatus for providing fallback assistanceinformation. The second apparatus may be one embodiment of the base unit110, the RAN node 210, and/or the RAN apparatus 600. The secondapparatus includes a transceiver that receives a first message from anetwork function in a first core network, wherein the first messageindicates service fallback of a remote unit connected to the RAN nodeand indicates a target CN. The second apparatus also includes aprocessor that determines service fallback parameters for the remoteunit. Moreover, the transceiver sends a connection release message tothe remote unit, the connection release message including redirectioninformation for the service fallback, the redirection informationincluding the target CN.

In some embodiments, the redirection information further includes atarget radio access technology (“RAT”). In certain embodiments, thefirst message additionally includes RAN node information, wherein theRAN node information comprises at least one of: UE security context andUE mobility restrictions.

In some embodiments, the first message includes a target RAT. In suchembodiments, determining the service fallback parameters includesselecting a fallback RAT based on the target RAT and one or more of:radio topology, and radio conditions of the remote unit.

In some embodiments, the first message includes target fallbackinformation, the target fallback information including one or more of: atarget radio access technology (“RAT”), the target CN, a target mobilitytype, a service request by the remote unit that triggered the servicefallback, and criticality requirements of a data connection of theremote unit.

Disclosed herein is a second method for providing fallback assistanceinformation. The second method may be implemented by the base unit 110,the RAN node 210, and/or the RAN apparatus 600. The second methodincludes receiving, at a RAN node, a first message from a networkfunction in a first core network, wherein the first message indicatesservice fallback of a remote unit connected to the RAN node andindicates a target CN. The second method includes determining servicefallback parameters for the remote unit and sending a connection releasemessage to the remote unit, where the connection release messageincludes redirection information for the service fallback. Here, theredirection information includes the target CN.

In some embodiments of the second method, the redirection informationfurther includes at least a target RAT. In certain embodiments, thefirst message additionally includes RAN node information, wherein theRAN node information includes at least one of: UE security context andUE mobility restrictions.

In some embodiments of the second method, the first message includes atarget RAT. In such embodiments, determining the service fallbackparameters includes selecting a fallback RAT based on the target RAT andone or more of: radio topology, and radio conditions of the remote unit.In certain embodiments, the first message includes target fallbackinformation, the target fallback information including one or more of: atarget RAT, the target CN, a target mobility type, a service request bythe remote unit that triggered the service fallback, and criticalityrequirements of a data connection of the remote unit.

Disclosed herein is a third apparatus for providing fallback assistanceinformation. The third apparatus may be one embodiment of the AMF 131and/or the network function apparatus 700. The third apparatus includesa transceiver that receives a service request from a remote unit,wherein the service request requires fallback to at least one of: adifferent RAT and a different CN. The third apparatus includes aprocessor that identifies at least one of: a target RAT and a target CN,based on at least one of: the requested service, remote unitcapabilities and network configuration. Moreover, the transceiverindicates, to a RAN node, the at least one of: a target RAT and a targetCN, based on the requested service, wherein the RAN node performsfallback procedure with the remote unit based on the at least one of: atarget RAT and a target CN. Here, performing fallback procedure includesone of:

In some embodiments, the service request indicates that the remote unitrequires emergency services fallback. In certain embodiments, indicatingthe at least one of: a target RAT and a target CN, comprises sending atleast one of: a prioritized list of target RATs and a prioritized listof CNs.

In some embodiments, the transceiver further sends a signaling messagewith fallback information to the RAN node, wherein the fallbackinformation includes at least one of: a target RAT, a target trackingarea, the request service, a mobility type, and service requirements ofa data connection used by the remote unit.

In such embodiments, the mobility type indicates idle mode mobility ofthe remote unit, the transceiver receives an acknowledgment message fromthe RAN node, and the processor, in response to the acknowledgmentmessage, performs one of: releasing a data connection used by the remoteunit, suspending a data connection used by the remote unit, and handingover a data connection used by the remote unit.

Disclosed herein is a third method for providing fallback assistanceinformation. The third method may be implemented by the AMF 131 and/orthe network function apparatus 700. The third method includes receiving,at a network function part of a first core network, a service requestfrom a remote unit, wherein the service request is for a service whichrequires fallback to at least one of: a different RAT and a differentCN. The third method includes identifying, at the network function, atleast one of: a target RAT and a target CN, based on at least one of:the requested service, remote unit capabilities and networkconfiguration. The third method includes indicating, to a RAN node, theat least one of: a target RAT and a target CN, based on the requestedservice, wherein the RAN node performs fallback with the remote unitbased on the at least one of: a target RAT and a target CN.

In some embodiments, the service request indicates that the remote unitrequires emergency services fallback. In certain embodiments, indicatingthe at least one of: a target RAT and a target CN, comprises sending atleast one of: a prioritized list of target RATs and a prioritized listof CNs.

In some embodiments, the third method includes sending a signalingmessage with fallback information to the RAN node, wherein the fallbackinformation includes at least one of: a target RAT, a target trackingarea, the request service, a mobility type, and service requirements ofa data connection used by the remote unit.

In such embodiments, the mobility type indicates idle mode mobility ofthe remote unit, wherein the method further includes receiving anacknowledgment message from the RAN node and performing an action inresponse to the acknowledgment message, the action being one of:releasing a data connection used by the remote unit, suspending a dataconnection used by the remote unit, and handing over a data connectionused by the remote unit.

Embodiments may be practiced in other specific forms. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A method comprising: sending, by a remote unit, aservice request, wherein the service request requires fallback to atleast one of: a different radio access technology (“RAT”) and adifferent system generation, wherein the service request indicates thatservice fallback is required; receiving, at the remote unit, aconnection release message, where the connection release messageincludes redirection information for the service fallback, theredirection information including a target RAT cell and a target system;selecting a non-access stratum (“NAS”) protocol from a plurality of NASprotocols supported by the remote unit, wherein the selection is basedon the target system; and connecting to the target RAT and the targetsystem using a procedure of the selected NAS protocol.
 2. The method ofclaim 1, wherein the procedure of the selected NAS protocol in thetarget system is one of: an evolved packet core (“EPC”) NAS procedureand a fifth-generation core (“5GC”) NAS procedure.
 3. The method ofclaim 1, where the procedure of the selected NAS protocol in the targetsystem comprises initiating the requested service via the target system.4. The method of claim 1, wherein sending the service request includesindicating at least one of: a type of the requested service and a radiocapability of the remote unit, wherein the radio capability comprises anindication of whether the remote unit is dual-transmission capable. 5.The method of claim 1, wherein the redirection information furtherincludes target cell information indicating at least one RAT.
 6. Themethod of claim 5, wherein the indicated RAT is evolved universalterrestrial radio access network (“E-UTRAN”), the method furthercomprising providing the target system to one or more upper layers inthe remote unit.
 7. A method comprising: receiving, at a radio accessnetwork (“RAN”) node, a first message from a network function in a firstcore network, wherein the first message indicates service fallback of aremote unit connected to the RAN node and indicates a target system;creating service fallback parameters for the remote unit, wherein thetarget system is used to execute one of: inter-RAT redirection andinter-system redirection, wherein inter-system redirection is triggeredif the target system is different than a current system of the remoteunit; and sending a connection release message to the remote unit, wherethe connection release message includes redirection information forservice fallback, the redirection information including a target radioaccess technology (“RAT”) cell and the target system, wherein the targetsystem is used to resolve the target RAT cell.
 8. The method of claim 7,further comprising: forwarding, by the RAN node, a service request tothe network function, the service request sent by the remote unit,wherein the service request indicates that service fallback is required.9. The method of claim 7, wherein the first message additionallycomprises RAN node information, wherein the RAN node informationcomprises at least one of: UE security context and UE mobilityrestrictions.
 10. The method of claim 7, wherein the first messagecomprises the target RAT, wherein determining the service fallbackparameters includes selecting a fallback RAT based on the target RAT andone or more of: radio topology, and radio conditions of the remote unit.11. The method of claim 7, wherein the first message comprises targetfallback information, the target fallback information including one ormore of: the target RAT, the target system, a target core network(“CN”), a target mobility type, a service request by the remote unitthat triggered the service fallback, and criticality requirements of adata connection of the remote unit.
 12. An apparatus comprising: atransceiver that receives a first message from the network function,wherein the first message indicates service fallback of the remote unitand indicates a target system; and a processor that creates servicefallback parameters for the remote unit, wherein the target system isused to execute one of: inter-RAT redirection and inter-systemredirection, wherein inter-system redirection is triggered if the targetsystem is different than a current system of the remote unit, whereinthe transceiver sends a connection release message to the remote unit,the connection release message including redirection information for theservice fallback, the redirection information including a target radioaccess technology (“RAT”) cell and the target system, wherein the targetsystem is used to resolve the target RAT cell.
 13. (canceled)
 14. Theapparatus of claim 12, wherein the first message includes the targetRAT, wherein determining the service fallback parameters includesselecting a fallback RAT based on the target RAT and one or more of:radio topology, and radio conditions of the remote unit.
 15. Theapparatus of claim 12, wherein the first message includes targetfallback information, the target fallback information including one ormore of: the target RAT, the target system, a target core network(“CN”), a target mobility type, a service request by the remote unitthat triggered the service fallback, and criticality requirements of adata connection of the remote unit.
 16. The apparatus of claim 12,wherein the first message additionally comprises RAN node information,wherein the RAN node information comprises at least one of: UE securitycontext and UE mobility restrictions.
 17. A method comprising:receiving, at a network function part of a first core network, a servicerequest from a remote unit, wherein the service request is for a servicewhich requires fallback to at least one of: a different radio accesstechnology (“RAT”) and a different system; identifying, at the networkfunction, at least one of: a target RAT and a target system, based on atleast one of the requested service, remote unit capabilities and networkconfiguration; and indicating, to a radio access network (“RAN”) node,the at least one of: a target RAT and a target-GN system, based on therequested service, wherein the RAN node performs fallback with theremote unit based on the at least one of: a target RAT and a targetsystem.
 18. The method of claim 17, wherein the service requestindicates that the remote unit requires emergency services fallback. 19.The method of claim 17, wherein indicating the at least one of a targetRAT and a target system comprising sending at least one of a prioritizedlist of target RATs and a prioritized list of systems.
 20. The method ofclaim 17, further comprising sending a signaling message with fallbackinformation to the RAN node, wherein the fallback information includesat least one of: a target RAT, a target tracking area, the requestservice, a mobility type, and service requirements of a data connectionused by the remote unit.
 21. The method of claim 20, wherein themobility type indicates idle mode mobility of the remote unit, themethod further comprising: receiving an acknowledgment message from theRAN node; and performing an action in response to the acknowledgmentmessage, the action being one of: releasing a data connection used bythe remote unit, suspending a data connection used by the remote unit,and handing over a data connection used by the remote unit. 22.(canceled)
 23. (canceled)
 24. (canceled)
 25. (canceled)
 26. (canceled)